1. Field of the Invention
The present invention relates to an ink composition, and an inkjet recording method employing same. More particularly, it relates to an ink composition, suitable for inkjet recording, that cures with high sensitivity upon exposure to actinic radiation and gives a cured material having sufficient flexibility after the ink is cured, and to an inkjet recording method.
2. Description of the Related Art
As image recording methods for forming an image on a recording medium such as paper based on an image data signal, there are an electrophotographic system, sublimation type and melt type thermal transfer systems, an inkjet system, etc. In the electrophotographic system, a process of forming an electrostatic latent image on a photosensitive drum by electrically charging and exposing is required, and the system is complicated; as a result, there is the problem that the production cost is high. With regard to the thermal transfer system, although the equipment is inexpensive, due to the use of an ink ribbon there is the problem that the running cost is high and waste material is generated.
On the other hand, with regard to the inkjet system, since an image is formed directly on a recording medium by discharging an ink only on a required image area using inexpensive equipment, the ink can be used efficiently and the running cost is low. Furthermore, there is little noise and it is excellent as an image recording system.
With regard to an ink composition that is curable upon exposure to actinic radiation such as UV rays (radiation curing type ink composition), for example, an inkjet recording ink composition, there is a demand for one that cures with high sensitivity and can form an image with high image quality. By achieving high sensitivity, high curability upon exposure to actinic radiation can be imparted, and there are therefore provided various benefits such as a reduction in power consumption, a longer lifetime of an actinic radiation generator due to a decrease in the load thereon and, due to sufficient curing being achieved, suppression of evaporation of uncured low-molecular-weight material and of a reduction in the strength of an image formed.
In recent years, the UV-curing inkjet system has been attracting attention since it has relatively low odor and can record on a recording medium that does not have rapid drying properties or ink absorbing properties.
As photopolymerization initiators used in such a photopolymerization curing type ink composition, benzil, benzoin, benzoin ethyl ether, Michler's ketone, anthraquinone, acridine, phenazine, benzophenone, 2-ethylanthraquinone, etc. are generally used (ref. e.g. Bruce M. Monroe et al., Chemical Reviews, Vol. 93, p. 435-448 (1993)). However, when these general photopolymerization initiators are used, since the curing sensitivity of the photopolymerizable composition cannot be said to be sufficient, a long period of time is required for image exposure in the formation of an image, and in the case of a detailed image, if there is slight vibration during an image formation step, there is the problem that the image cannot be reproduced with good image quality. Moreover, since it is necessary to increase the total amount of energy radiated from an exposure light source, it is necessary to take into consideration the radiation of a large amount of heat generated thereby.
A method has already been proposed in which a sensitizing dye is used together with a photopolymerization initiator so as to enhance the sensitivity of a radiation curing type polymerizable compound toward radiation, and the use of various types of polymerization initiator systems has been disclosed. For example, use of a thioxanthone-based compound as a sensitizing dye has been proposed (JP-A-6-308727 and JP-A-56-143202 (JP-A denotes a Japanese unexamined patent application publication)). However, since a sensitizing dye such as a thioxanthone-based compound remains as a monomer without bonding to another compound in a cured coating after a polymerizable composition is cured, it tends to easily cause the problems of migration toward another material that is in contact with the cured coating, behaving in the same manner as a plasticizer in the cured coating due to it being a low molecular weight component to thus degrade the coating strength, or causing tackiness on the cured coating surface to thus degrade surface curing sensitivity.
In this technical field, in which an ink composition is utilized, one of the important issues is to suppress blocking between an image formed on an ink receiving layer and another material that is in contact with the image, that is, a situation caused by ink transfer from the image to the other material or undesired adhesion to the other material due to the ink. Because of this, the application of a polyfunctional thioxanthone compound having excellent anti-blocking properties to an ink composition has been proposed (e.g. JP-PCT-2005-512973 (JP-PCT denotes a published Japanese translation of a PCT application)), but the ink composition thus obtained has high viscosity and is not suitable as an inkjet ink.